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24-bit word-oriented computing, some thoughts http://anycpu.org/forum/viewtopic.php?f=3&t=576 |
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Author: | BigEd [ Sat Apr 13, 2019 6:47 pm ] |
Post subject: | Re: 24-bit word-oriented computing, some thoughts |
Revaldinho did some analysis of quick syntheses (not optimal syntheses) of three OPC cores which, as it happens, have different word widths but also somewhat different instruction sets: Quote: Code: Bits FFs Slices LUTs MHz OPC6 16 82 109 356 94 OPC8 24 130 121 415 172 OPC7 32 135 212 565 145 Attachment:
File comment: Comparing 3 OPC machines image(1).png [ 5.91 KiB | Viewed 6070 times ] Both OPC8 (24 bit) and 7 (32 bit) come out running at higher frequency than OPC6 (16 bit). There are a few reasons for this:
Comparing OPC7 and 8
And then of course OPC8 has a 24 bit word rather than a 32 bit one, although that doesn't really shorten the carry chain in add/subtract. So, on the face of it OPC8 looks very good. Will be interesting to see if OPC6 has made up for that slower clock rate with better cycle efficiency by being able to skip EAD on some (definitely not all) instruction types when comparing the traces. Beware: OPC6, 7 and 8 are named in order of invention, which is unfortunately not in order of word width. This can cause confusion, or at least, typos. Edit: fixed a typo where OPC7 should have been OPC8. (!!) |
Author: | joanlluch [ Sat Apr 13, 2019 10:06 pm ] |
Post subject: | Re: 24-bit word-oriented computing, some thoughts |
Hi all, Please, what do the acronyms "FFs", "Slices", "LUTs" and "EAD" stand for? I'm sure I know them all and it's only a language issue from my part (I'm not English native), so just expanding these acronyms to their complete words or providing a very simple definition will suffice. Sorry for briefly interfering the normal conversational flow, and thanks in advance! Joan |
Author: | robfinch [ Sun Apr 14, 2019 2:36 am ] |
Post subject: | Re: 24-bit word-oriented computing, some thoughts |
FF’s stands for flip-flops which are also called registers. Whenever I see FF I think of a 74LS74 ‘D’ flip-flop. Slices are a logic unit within an FPGA. An FPGA is composed of many slices. Slices typically contain multiple FF’s and LUT’s. Also similar are CLB’s which stands for configurable logic blocks. LUT’s stands for look-up-table. EA typically stands for effective-address. So EAD is a word associated with generating the effective address. |
Author: | BigEd [ Sun Apr 14, 2019 6:43 am ] |
Post subject: | Re: 24-bit word-oriented computing, some thoughts |
Indeed, FFs, slices and LUTs are all measures of the cost of the design in terms of resources on the FPGA. FFs are the state and LUTs are the logic, whereas each slice contains a number of both of those, and might be partially used. Different FPGA families have different LUT designs and different slice designs, so the counts of these may not be comparable across families, but within a family we can compare the cost of implementations by using these counts. EAD is a pipeline stage: it's the stage which deals with the optional operand word, which either gives an Effective Address or gives some literal Data. (I think one could instead call it the Operand stage.) See this post for more on the pipeline: viewtopic.php?p=3026#p3026 Edit: sorry about the obfuscation! Edit: perhaps EAD is best described as a system state, not a pipeline stage. |
Author: | joanlluch [ Sun Apr 14, 2019 8:54 pm ] |
Post subject: | Re: 24-bit word-oriented computing, some thoughts |
Thank you very much for the acronym definitions. It's now suddenly much clearer to me, and I understand what you are talking about. I appreciate it. |
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